A breathtaking view on the bowels of a volcano

Volcanoes are made up of a complex arrangement of fractured rocks and pockets of liquid and gas that are so heterogeneous that imaging them is dramatically complicated. Seismic tomography exploits earthquakes to probe their mechanical properties, but it requires significant seismic activity and the resolution of the images obtained is only of the order of a few kilometers. Researchers at the Institut Langevin (CNRS/ESPCI Paris – PSL University) and the Institut de physique du globe de Paris (Université Paris Cité/CNRS) have developed a new imaging method, known as passive matrix imaging, which dives into the bowels of the volcano to a depth of ten kilometers and resolves its internal plumbing with a precision of the order of a hundred meters based on seismic noise alone. These results were obtained on La Soufrière volcano in Guadeloupe. At shallow depth, this image reveals the tortuous shape of the volcano’s conduit. Beyond a 5 km-depth, they confirm the existence of a large magma storage zone and its arrangement into a network of interconnected horizontal magma lenses. Besides confirming previous conceptual models proposed in the literature, such images offer a unique viewpoint on volcanoes, capable of revolutionizing the field of volcanology.

To achieve this, the scientists, in collaboration with the Volcanological and Seismological Observatory of Guadeloupe (OVSG-IPGP), deployed a sparse network of geophones, which capture not only the strong tremors of earthquakes, but also the seismic noise induced by wind, ocean and human activity. This seismic noise, measured over a period of two months, has been leveraged to construct a reflection matrix, inspired by previous work by the same team on ultrasound imaging and optical microscopy, recently published in Nature Communications. This matrix is used to finely compensate for the distortions that seismic waves undergo as they pass through the volcano’s various geological structures and magma storage area. These heterogeneities are no longer an obstacle and an image of the volcano’s internal structure is obtained as if the volcano had become transparent.